13.3
Logic Systems

Digital signals are actually analog voltage (or current) levels that vary continuously as they change.
Digital simulation
assumes that digital signals may only take on a set of
logic values
(or
logic states
—here we will consider the two terms equivalent) from a
logic system
. A logic system must be chosen carefully. Too many values will make the simulation complicated and slow. With too few values the simulation may not accurately reflect the hardware performance.

A
two-value logic system
(or
two-state logic system) has a logic value
'0'
corresponding to a
logic level
'zero' and a logic value
'1'
corresponding to a logic level 'one'. However, when the power to a system is initially turned on, we do not immediately know whether the logic value of a flip-flop output is
'1'
or
'0'
(it will be one or the other, but we do not know which). To model this situation we introduce a logic value
'X'
, with an unknown logic level, or
unknown
. An unknown can
propagate
through a circuit. For example, if the inputs to a two-input NAND gate are logic values
'1'
and
'X'
, the output is logic value
'X'
or unknown. Next, in order to model a three-state bus, we need a
high-impedance state
. A high-impedance state may have a logic level of 'zero' or 'one', but it is not being driven—we say it is floating. This will occur if none of the gates connected to a three-state bus is driving the bus. A
four-value logic system
is shown in
Table 13.2
.

TABLE 13.2
A four-value logic system.

Logic state

Logic level

Logic value

0

zero

zero

1

one

one

X

zero or one

unknown

Z

zero, one, or neither

high impedance

13.3.1
Signal Resolution

What happens if multiple
drivers try to drive different logic values onto a bus?
Table 13.3
shows a
signal-resolution function
for a four-value logic system that will predict the result.

TABLE 13.3
A resolution function R {A, B} that predicts the result of two drivers simultaneously attempting to drive signals with values A and B onto a bus.

R {A, B}

B = 0

B = 1

B = X

B = Z

A = 0

0

X

X

0

A = 1

X

1

X

1

A = X

X

X

X

X

A = Z

0

1

X

Z

A
resolution function, R {A, B}, must be
commutative
and
associative
. That is,

R {A, B} = R {B, A} and R {R {A, B}, C} = R {A, R {B, C}}.

(13.4)

R {A, B} = R {B, A} and R {R {A, B}, C} = R {A, R {B, C}}.(13.4)

Equation
13.4
ensures that, if we have three (or more) signals to resolve, it does not matter in which order we resolve them. Suppose we have four drivers on a bus driving values
'0'
,
'1'
,
'X'
, and
'Z'
. If we use
Table 13.3
three times to resolve these signals, the answer is always
'X'
whatever order we use.

13.3.2
Logic Strength

In CMOS logic we use
n
-channel transistors to produce a logic level 'zero' (with a forcing strength) and we use
p
-channel transistors to force a logic level 'one'. An
n
-channel transistor provides a weak logic level 'one'. This is a new logic value, a
resistive 'one'
, which has a logic level of 'one', but with
resistive strength
. Similarly, a
p
-channel transistor produces a
resistive 'zero'
. A resistive strength is not as strong as a forcing strength. At a high-impedance node there is nothing to keep the node at any logic level. We say that the logic strength is
high impedance
. A high-impedance strength is the weakest strength and we can treat it as either a very high-resistance connection to a power supply or no connection at all.

TABLE 13.4
A 12-state logic system.

Logic level

Logic strength

zero

unknown

one

strong

S0

SX

S1

weak

W0

WX

W1

high impedance

Z0

ZX

Z1

unknown

U0

UX

U1

With the introduction of logic strength, a logic value may now have two properties: level and strength. Suppose we were to measure a voltage at a node
N
with a digital voltmeter (with a very high input impedance). Suppose the measured voltage at node
N
was 4.98 V (and the measured positive supply,
V
DD
= 5.00 V). We can say that node
N
is a logic level 'one', but we do not know the logic strength. Now suppose you connect one end of a 1 k
W
resistor to node
N
, the other to GND, and the voltage at
N
changes to 4.95 V. Now we can say that whatever is driving node
N
has a strong forcing strength. In fact, we know that whatever is driving
N
is capable of supplying a current of at least 4.95 V / 1 k
W
⊕ 5 mA. Depending on the logic-value system we are using, we can assign a logic value to
N
. If we allow all possible combinations of logic level with logic strength, we end up with a matrix of logic values and logic states.
Table 13.4
shows the 12 states that result with three logic levels (zero, one, unknown) and four logic strengths (strong, weak, high-impedance, and unknown). In this logic system, node
N
has logic value
S1
—a logic level of 'one' with a logic strength of 'strong'.

The
Verilog logic system
has three logic levels that are called
'1'
,
'0'
, and
'x'
; and the eight logic strengths shown in
Table 13.5
. The designer does not normally see the logic values that result—only the three logic levels.

TABLE 13.5
Verilog logic strengths.

Logic strength

Strength number

Models

Abbreviation

supply drive

7

power supply

supply

Su

strong drive

6

default gate and assign output strength

strong

St

pull drive

5

gate and assign output strength

pull

Pu

large capacitor

4

size of trireg net capacitor

large

La

weak drive

3

gate and assign output strength

weak

We

medium capacitor

2

size of trireg net capacitor

medium

Me

small capacitor

1

size of trireg net capacitor

small

Sm

high impedance

0

not applicable

highz

Hi

The
IEEE Std 1164-1993 logic system defines a variable type,
std_ulogic
, with the nine logic values shown in
Table 13.6
. When we wish to simulate logic cells using this logic system, we must define the primitive-gate operations. We also need to define the process of
VHDL signal resolution
using
VHDL signal-resolution functions
. For example, the function in the IEEE Std_Logic_1164 package that defines the
and
operation is as follows
1
:

TABLE 13.6
The nine-value logic system, IEEE Std 1164-1993.

Logic state

Logic value

Logic state

Logic value

'0'

strong low

'X'

strong unknown

'1'

strong high

'W'

weak unknown

'L'

weak low

'Z'

high impedance

'H'

weak high

'-'

don’t care

'U'

uninitialized

function "and"(l,r : std_ulogic_vector) return std_ulogic_vector is

alias lv : std_ulogic_vector (1 to l'LENGTH ) is l;

alias rv : std_ulogic_vector (1 to r'LENGTH ) is r;

variable result : std_ulogic_vector (1 to l'LENGTH );

constant and_table : stdlogic_table := (

-----------------------------------------------------------

--| U X 0 1 Z W L H - | |

-----------------------------------------------------------

( 'U', 'U', '0', 'U', 'U', 'U', '0', 'U', 'U' ), -- | U |

( 'U', 'X', '0', 'X', 'X', 'X', '0', 'X', 'X' ), -- | X |

( '0', '0', '0', '0', '0', '0', '0', 'U', '0' ), -- | 0 |

( 'U', 'X', '0', '1', 'X', 'X', '0', '1', 'X' ), -- | 1 |

( 'U', 'X', '0', 'X', 'X', 'X', '0', 'X', 'X' ), -- | Z |

( 'U', 'X', '0', 'X', 'X', 'X', '0', 'X', 'X' ), -- | W |

( '0', '0', '0', '0', '0', '0', '0', '0', '0' ), -- | L |

( 'U', 'X', '0', '1', 'X', 'X', '0', '1', 'X' ), -- | H |

( 'U', 'X', '0', 'X', 'X', 'X', '0', 'X', 'X' ), -- | - |);

begin

if (l'LENGTH /= r'LENGTH) then assert false report

"arguments of overloaded 'and' operator are not of the same

length"

severity failure;

else

for i in result'RANGE loop

result(i) := and_table ( lv(i), rv(i) );

end loop;

end if;

return result;

end "and";

If
a = 'X'
and
b = '0'
, then
(a and b)
is
'0'
no matter whether
a
is, in fact,
'0'
or
'1'
.